The present invention relates to an optical element forming method and more particularly to a method of forming an economical optical element of high precision as to shape and surface and suitable for mass-production.
To produce an optical element such as a lens, a prism or the like, there is recently known, instead of a method of grinding an optical element blank such as glass, plastic or the like, a method of previously preparing an optical element blank in a predetermined shape, feeding this blank in a forming mold, heating and then forming the blank under pressure (for example, Japanese Laid-Open Patent Publication No. 58-84134). As to such an optical element blank to be used for forming under pressure, the shape thereof is desired to be as simple as possible in view of workability and cost. As a typical example, there is proposed a forming method with the use of a column-like blank (for example, Japanese Laid-Open Patent Publication No. 3-50126).
In a conventional forming method, the forming operation has been conducted while eliminating gas in a closed space between the blank and the mold by alternately repeating pressurizing and stopping the application of pressure. However, such a method presents the problems that the forming precision is unstable, resulting in low yield, and that predetermined precision is reached only with difficulty. Further, to obtain an optical element having a predetermined center thickness Y and a predetermined optically effective surface diameter .o slashed.EFF, there has been used a column-like blank of which center thickness X is greater than Y and of which both end surfaces and lateral surface are mirror-finished. This disadvantageously increases the cost of the optical element.